Wave packets, group velocities and rays in lossy media, revisited

This study, which also serves as an introduction and a review, was mainly prompted by recent discussions concerning the group velocitiy concept in absorptive media, and the "correctness" of Maxwell's equations, initiated by Harmuth's work. In spite of some derisive comments about the intuitive "textbook" concepts this is exactly the approach used here to develop a consistent definition of the group velocity in absorptive media. It is well-known that the definition of group velocity in absorptive media is not unique. Here we adhere to the hypothesis that in order to describe the transport of energy parcels through real space, the group velocity must be real. Numerical examples are displayed, and theoretical and experimental aspects are briefly discussed. The arguments show that a physically meaningful definition of the group velocity in absorptive media is consistent with the Fermat principle and the special relativistic limitation on the speed of light. Due to space restrictions we present here an abbreviated version.